The present invention relates in general to detecting the baseline shift artifact which contaminates an electrocardiographic signal and more particularly concerns novel apparatus and techniques for detecting that portion of the baseline shift artifact caused by capacitive coupling of charged bodies to the electrodes while retaining the advantages of eliminating the effects of baseline shift artifact resulting from the motion of the electrodes as disclosed in prior copending application Ser. No. 754,538, filed Dec. 27, 1976, now U.S. Pat. No. 4,112,930, and application Ser. No. 917,877 filed June 22, 1978, and entitled APPARATUS AND METHOD FOR EGG BASELINE SHIFT DETECTING.
Although most baseline shift artifact which contaminates an electrocardiographic signal results from motion of the electrodes and the effect of that component is sharply reduced by the inventions disclosed in the aforesaid copending applications, capacitive coupling of external bodies to the electrode wires is a source of additional baseline shift artifact. Patients' clothes, particularly when they are made of synthetic materials that readily develop a substantial static charge, are the most common source of undesired baseline shift artifact capacitively coupled to the electrode wires. This phenomenon of capacitive coupling of charged bodies to the electrodes is particularly severe when patients are much more mobile than patients lying in bed, hard-wired to a bedside monitor.
The voltage generated by the capacitively coupled artifact is directly proportional to the signal source impedance, typically the sum of two electrode-to-skin interfaces plus the impedance produced by the body. Although the impedance is typically less than 50,000 omhs when the electrodes are carefully installed so that the magnitude of the capacitively coupled artifact is correspondingly low, source impedances of 100,000 ohms and greater are not uncommon with a correspondingly higher capacitively coupled artifact.
Accordingly, it is an important object of this invention to provide means and method for detecting capacitively coupled baseline shift artifact.
It is a further object of the invention to achieve the preceding object with apparatus that is relatively easy and inexpensive to fabricate and install.
It is still a further object of the invention to achieve one or more of the preceding objects while maintaining the advantages of the invention disclosed in the aforesaid copending applications.